Recently, various types of materials have been studied as electrode material for energy storage devices. Among them, lithium titanate is attracting attention for use as an active material of an energy storage device for electric vehicles such as HEV, PHEV, and BEV for its superior input-output performance when used as an active material.
However, particularly when used for an HEV, since energy discharged during slowdown of an electric vehicle is recovered (called “regeneration”) and extremely large current instantaneously flows to an energy storage device, further improvement in quick charge characteristics is desired. Further, since vehicles are used for more than ten years, it is also desired that the energy storage device for an electric vehicle maintains its performance that does not deteriorate over a long period of time.
Patent Document 1 discloses that by making an average pore diameter of lithium titanate within a range of 50 to 500 Å and making fine pore volume 0.01 ml/g or more, charge discharge rate performance and cycle characteristics become better.
Patent Document 2 discloses that, by including 0.10 to 0.25% by mass of K2O and 0.10 to 0.50% by mass of P2O5, an initial discharge capacity of lithium titanate becomes larger. When doing this, in order to adjust K2O and P2O5 contents in lithium titanate, specific amounts of K2O and P2O5 are included in raw material of titanium oxide, such titanium oxide and lithium hydroxide are mixed so as to make an atomic ratio of Li to Ti 4:5, and calcined at 875° C. or 800° C. in the air for 6 hours to prepare lithium titanate. More, in Patent Document 2, titanium oxide as raw material is prepared by adding predetermined contents of K2O and P2O5 to hydrous titanium oxide, calcining, and milling the same. That is, in Patent Document 2, P2O5 is used as the raw material to introduce P atoms.
Patent Document 3 discloses that, by allowing coexistence of Nb, K and P in lithium titanate, solubility of Nb accelerates and at the same time, necking between particles is suppressed by an effect of Nb addition, as a result, particle growth in lithium titanate is suppressed, so as that aggregation hardly occurs, and thus lithium titanate having better charge/discharge rate performance can be obtained. In the case, the raw material used to adjust concentration of P atoms is ammonium dihydrogen phosphate, and lithium titanate is prepared by mixing lithium carbonate and titanium oxide so as to make a molar ratio of Li:Ti in the product obtained after calcination 4:5 (also, a predetermined amount of potassium hydroxide and niobium oxide are mixed), calcining the same at 820° C. or 850° C. for three hours in the air. In Patent Document 3, P content in lithium titanate is 0.013 to 0.240% by mass.